Sound reproducing apparatus



Sept. 27, 1966 BRYAN T L 3,275,758

SOUND REPRODUC ING APPARATUS Filed Sept. 27, 1962 5 sheets-$11861, 1

0 39 E 80 OPEN 32/ U1 6 e0 40 Z 0 4 no S7. 8% 20 2E l I l l a 3 5 E3FREQUENCY IN excuse/sec.

INVENTORS SAMUEL BRYAN WALTER B. UDELL S A/$5 6W ATTORNEY 2/ Fig. 2

Sept. 27, 1966 Filed Sept. 27, 1962 Distortion of I30 Cycles/ Second s.BRYAN ETAL 3,275,758

SOUND REPRODUCING APPARATUS 5 Sheets-Sheet 2 D at R Open Circuited D ofR Short Circuited 500m Frequency In Cycles/Second SAMUEL BRYAN WALTER B.UDELL ATTbRNEY Sept. 27, 1966 s. BRYAN ETAL 3,275,753

SOUND REPRODUCING I APPARATUS Filed Sept. 27, 1962 5 Sheets-Sheet 5 iiiFig, 10

R t ohms constant Relative Phase Anglein Degrees 8 8 40 6O 80 I00 I20I40 I60 Frequency in Cycles /Second Fig. l/ 23 9 lao- Q= C .s 60- Um 8L' S4on I I F! I I l f I l l 40 5O 6O 90 I00 I10 I I I I Frequency inCycles/Second VENTQRfi SAMUEL BRYAN WALTER B.UDELL Mew ATTORNEY UnitedStates Patent 3,275,758 SOUND REPRODUCING APPARATUS Samuel Bryan, SilverSpring, Md., and Walter B. Udell, Philadelphia, Pa, assignors, by mesneassignments, to Walter B. Udell, Philadelphia, Pa.

Filed Sept. 27, 1962, Ser. No. 226,703 14 Claims. (Cl. 179-416) Thisinvention relates generally to sound reproducing apparatus, and moreparticularly relates to loudspeaker systems of the acoustical resonatorphase inverter type. The acoustical resonator is early described in onepractical form in the United States patent to Bobb, No. 2,059,- 929, asa device for substantially reducing the low frequency resonancesassociated with a loudspeaker mounted in an openbacked cabinet tothereby improve the distortion characteristics of the loudspeakersystem. Basically, the resonators described may be considered asloudspeakers without a voice coil and magnet structure in which the conemovements induced at the mechanically resonant frequencies are dampedout by viscous structures coupled to the apex of the cone. Theresonators are of course energy absorbing devices operative within apredetermined frequency band to materially lower the Q of the mechanicalsystem resonances of the loudspeaker system. In general, and asdescribed in the aforesaid patent, several resonators of appropriatelyselected resonant frequencies and Qs are required to properly damp abroadly resonant system or one exhibiting spaced resonance peaks.

This very same type of acoustical resonator in a closed cabinet has beenmore recently treated by Olson, in his book entitled, AcousticalEngineering, in which it is pointed out that this arrangement,designated as a drone cone, is superior to the phase inverter orHelmholtz resonator type of enclosure commonly known as the bass reflexcabinet. In general, the advantages are said to derive from the factthat the effective port area can be as large as that of the active ordriving cone, which is much larger than that possible with an open port,and that the phase and amplitude of particle motion are the same overthe entire drone cone, a condition which is not even approximated by anopen port. As a consequence the drone cone system is capable of widerfrequency range with greater acoustical output than is possible with anopen port because a lower particle velocity at higher pressure resultstogether with substantial reduction of the losses incurred due to phaseshift across the open port.

The present invention differs from the resonator systems above describedin that while the latter are strictly passive mechanical dissipationsystems, the present invention provides selectively controllable dynamicdamping of the driving cone which results in a marked reduction ofdistortion as compared with that of the passive dissipation systemstogether with augmented bass frequency outputs. Accordingly, it is aprimary object of this invention to provide a novel loudspeaker systemwhich incorporates therein a driving cone and acoustical resonatormechanically resonant throughout the resonant frequency range of thesystem, with the acoustical resonator also providing electro dynamicdamping of the driving cone.

Another object of this invention is to provide a novel loudspeakersystem as aforesaid wherein only a single resonator is employed whichhas the same mechanical resonance spectrum and Q as the driving cone.

Still another object of this invention is to provide a novel loudspeakersystem employing an acoustical resonator in an electro-acoust-icalvelocity sensitive feedback loop with the driving cone of the systemwhich varies the damping on the driving cone in accordance with thevelocity of the amplitude excursions of the latter.

Yet another object of this invention is to provide a Patented Sept. 27,1966 ice novel loudspeaker system as aforesaid which includes means forselectively altering the phase characteristic of the resonator to changethe phase angle difference between the driving cone and the resonator.

A further object of the invention is to provide a novel loudspeakersystem of the phase inverter type in which the system may be tunedelectrically to selectively alter the acoustic output thereof.

The foregoing and other objects of the invention will become clear froma reading of the following specification in conjunction with anexamination of the appended drawings, wherein:

FIGURE 1 illustrates in perspective a loudspeaker system according tothe invention with the enclosure back removed to reveal a loudspeakerand a resonator mounted therein;

FIGURE 2 is a front elevation of the loudspeaker system of FIGURE 1 .aswould be seen when viewed along line 2-2 thereof;

FIGURE 3 is a horizontal sectional view through the loudspeaker systemof FIGURE 2 as would be seen when viewed along line 33 thereof, theloudspeaker and resonator being shown in elevation;

FIGURE 4 is an axial diametric section taken through a typical dynamicloudspeaker of the permanent magnet field type;

FIGURE 5 is an enlarged fragmentary view of the corrugated edge surroundof the loudspeaker cone of FIG- URE 4- enclosed in the phantom circlethereof;

FIGURE 6 is a graph of distortion generation variation with change inresonator resistive electrical damping for two power levels at aconstant frequency;

FIGURE 7 is a graph showing the distortion reduction achievable with thedynamically damped resonators according to the invention as comparedwith the previously known passive resonators.

FIGURE 8 is a graph of relative phase shift versus frequency between thedriving loudspeaker cone and the resonator device for two differentconditions of operation;

FIGURE 9 is a representational showing of a resonator according to theinvention in a modified electrical circuit;

FIGURE 10 is a somewhat linearized graph of relative phase shift betweenthe loudspeaker cone and the resonator as a function of frequencyresulting from utilization of the circuit of FIGURE 9; and

FIGURE 1'1 is a plot of capacitance variation in the circuit of FIGURE 9as a function of frequency for three selected degrees of constant phaseshift between the cone of the driving loudspeaker and that of theresonator.

In the several figures, like elements are denoted by like referencecharacters.

Turning now to the drawings consider first FIGURES l to 5 in which therewill be seen a loudspeaker system including an enclosure 20 having anapertured front wall 21 to which are mounted in peripherally sealedfashion a loudspeaker 2'2 and a resonator 23, the resonator being inactuality another loudspeaker of the same kind as the loudspeaker 22.The voice coil terminals of a loudspeaker 22 are connected to a pair ofexternal terminals 24 mounted to the enclosure side wall 25 by a pair ofleads 26. The voice coil terminals 27 of resonator 23 are connected in aclosed loop through rheostat Rs by leads 28, the rheostat beingphysically mounted to the enclosure top wall 29 with its shaftprojecting therethrough and terminated externally by knob 30. Theinterior of the enclosure is suitably padded, although this is not shownfor purposes of clarity, and the enclosure is sealed up by securing theback 31 to the top, bottom and side walls of the enclosure 20 in anyconvenient manner to provide a substantially airtight interior chamber.

As seen in FIGURE 4 the resonator 23, and of course the active driver 22also, is a permanent magnet dynamic loudspeaker having a field magnet 32formed to set up a radial magnetic field Within which the turns of avoice coil 33 wound on cylindrical tube 34 are disposed for axial motiontransverse to the lines of magnetic flux of the field. The voice coiltube 34 is secured to the apex of cone 35 which latter is currugated atits outer periphery as best seen at 36 in FIGURE and is peripherallyclamped to the rim of the speaker frame by annular compression gasket37. The number, depth and flexibility of the corrugations 36 and thespider (not shown) determine the compliance or acoustical capacitance ofthe loudspeaker, while the mass of the cone 35 and voice coil structure33-34 determine the acoustical inductance of the loudspeaker.

It should be recognized at this point that the resonator according tothis invention diflers from the passive resonators described by theprior art in that it includes a voice coil and magnet structure and doesnot utilize viscous mechanical damping elements other than the inherentmechanical dissipative damping provided by the 'cone surround 36 andspider. With the voice coil 33 open circuited, the resonator 23 behavesmerely as a mechanically resonant system on which the magnet 32 has noeffect whatever, and is as a practical matter not present. The vibratorysystem of the resonator 23 is mechanically resonant over a limitedfrequency band tional to the amplitude of the driving cone oscillations.

The mechanical suspension system of the cone 35 does of course providedamping which is a function of excursion of the cone, relatively littledamping occurring at small excursions with progressively increasingresistance to larger excursions. The reason for this is that thesuspension system is relatively compliant at center position, becomingless and less compliant with displacement from center position. This isof course the same way in which the passive type of resonator behaves,the damping provided by the damping elements of the resonator increasingwith increased displacement of the resonator cone from its centerposition.

It is at this point that the operation of the resonator according to theinvention departs from the operation of previously known passiveresonators because the voice coil 33 is in fact not left in anopen-circuited condition, but is instead connected in a closed seriesloop with network elements which cause the resonator to become avelocity responsive damping device and to thus anticipate largeamplitude excursions before large displacements occur and react quicklyto dynamically damp the same while the excursion is still of relativelysmall amplitude. The passive type of resonator is incapable of thisaction because it is not velocity responsive, but responds only toamplitude changes which must first occur.

The resonator action is analogous to dynamic braking of an electricmotor and takes place because the induced vibrations of the resonatorcone drive the voice coil 33 across the lines of magnetic flux in theair gap of field magnet 32. A motional is thus generated which appearsacross voice coil terminals 27 and is of such polarity as to tend toproduce a current giving rise to a thrust on the voice coil whichopposes the motion which produces it, in accordance with Lenzs law. Theamplitude of the motional is of course proportional to the fieldstrength in the gap and to the velocity of voice coil motion. With thevoice coil terminals 27 open circuited there is no current flow and noelectromagnetic counterforce to the mechanically induced vibrations isgenerated. When, however, the voice coil circuit is closed throughnetwork elements to be described, a current flows which brakes themotion of the resonator cone and the driving cone by converting themechanical energy of cone motion into electrical energy and thendissipating the electrical energy by doing work in creating a magneticfield and driving current through resistive elements.

As will be subsequently seen in more detail the relative phase betweenmotion of the driving cone and the resonator cone is controllable withinlimits by proper choice of the network elements connected in closedcircuit with the resonator voice coil 33. This is an important featureof the invention for the following reason. It will be recalled that ithas been shown by others that one of the major advantages of the passivedrone cone phase inverter over the open port inverter is that the phaseof particle motion is the same over the entire drone cone. While this istrue it is not necessarily of any real value unless the phase ofparticle motion is that which is desired relative to the particle motionof the driving cone. For this reason careful consideration must normallybe given to the design of passive resonators and to the enclosures withwhich they are to be used. In particular the volume of the enclosuremust be chosen sufiiciently large to provide the required acousticalcapacitance to produce the desired phased output from the drone cone. Toa considerable extent the present invention provides the ability toproperly phase the resonator with enclosures of substantially reducedsize, and provides the ability to electrically tune a loudspeaker systemin an enclosure rather than requiring the physical reconstruction of theenclosure to vary the acoustical capacitance thereof.

FIGURES 6, 7 and 8 show the results achieved by resistance loading ofthe resonator voice coil as a function of the magnitude of Rs, whileFIGURES 9, 10 and 11 relate to phase control of the resonator cone bythe introduction of reactive elements into the resonator voice coilcircuit. The data illustrated was obtained by measure ments made with aloudspeaker system of the physical type shown in FIGURES 1 to 3 andhaving the following characteristics.

Enclosure internal dimensions=27" x 14" x 4 /2" Enclosure internalvolume with loudspeakers 0.8"ft. Driver 22 and resonator 23:

Jensen Mfg. Co. type P12NL loudspeaker 12" nominal diameter 8-ohmnominal voice coil impedance Free air cone resonance -30 cycles/ secondSystem resonance cycles/second.

It should be noted that the system resonance is substantially higherthan the free air cone resonance of the driver 22 and resonator 23 dueto the extremely small internal volume of the enclosure which was chosenfor several reasons. First, because small volume enclosures are comingincreasingly into public demand and improved results achieved with suchenclosures are ofimmediate interest to the general public. Second,because the use of a small enclosure raises the system resonance to afrequency where the radiation resistance of the driver is realtivelygood so that effective air loading of the driver cone is obtainedthrough the resonance region. This latter consideration makes itfeasible to electrically tune the resonator voice coil circuit below thesystem resonance to provide better in-phase coupling of the resonatorcone to the driver cone, resulting in better loading of the driver coneat lower frequencies which produces augmented bass response and lowerdistortion.

FIGURE 6 illustrates schematically the electrical circuit of theresonator voice coil which comprises the closed series loop includingvoice coil inductance L, voice coil resistance R motional generator 38and rheostat Rs. The magnitude and phase of the current pro duced in thevoice coil circuit by generator 38 is variable within limits determinedby the magnitude of Rs, which latter can be varied between a highresistance value and short circuit. The effect of so varying Rs isplotted for a signal of 130 cycles/second at constant power inputs tothe driver 122 of milliwatts and 500 milliwatts.

The high resistance values of Rs prevent any appreciable current flow inthe voice coil circuit and simulate the condition of the open-circuitedvoice coil wherein the only damping operative is that due to themechanical re sistances of the cone suspension and is comparable to thepassive type of resonator. The short circuit condition of Rs permits asubstantial current to flow in the voice coil circuit, which currenteffects heavy electromagnetic damping on the resonator cone and hence onthe driving cone. The significant reduction in total generateddistortion which results is shown in the graph of FIGURE 6 together withthe variation in distortion as a function of Rs between its shortcircuit and effectively open circuit conditions.

The graph of FIGURE 7 compares the total distortion generated by theloudspeaker system when the resonator voice coil is open circuited withthat generated when the voice coil is short circuited over the frequencyrange of mechanical resonance of the resonator for three levels ofelectrical power input to the voice coil of driver 22. The open circuitdistortion is in all cases higher than the short circuit distortion, andfor the 150 milliwatt curve,- which is a normal average power inputlevel to be expected in a residential sound system it is seen that theopen circuit or passive resonator distortion at 130 cycles is 55% higherthan the short circuit or dynamically damped resonator distortion. Thehigher power curves show the same general condition but to alesserdegree as the power input to the driver is increased. Thesignificance of the curves of FIGURE 7 is not that there is asignificant distortion peak at the resonant frequency of the system, butthat a material improvement in the distortion characteristics ofloudspeaker systems can be effected by dynamic damping, and that thisimprovement over the passive type of resonator is effective underconditions where a current flows in the resonator voice coil.

FIGURE 8 shows the effective phase shift between the cone motion of thedriver 22 and the cone motion of the resonator 23 for an open-circuitedresonator voice coil and with the voice coil shunted by ten ohms, theline 39 designating the open-circuit phase and the line 40 designatingthe resistive shunt phase. The improvement in phase response of theresonator with its voice coil shunted is obviously not due to a speed-upof resonator cone motion since the effect of the induced current is toproduce a counter motion of the cone, but is in fact due to the heavydamping produced on the driver cone which prevents it from excursing tothe extent formerly possible without the electromagnetic damping of theresonator. It should also be noted that the slope of line 40 is lessthan that of line 39 so that the phase shift with the shunted resonatorvoice coil is more gradual. The slope of line 40 may be considerablylowered by decreasing the voice coil shunt resistance, thus furtherimproving the phase characteristic.

Turning now to FIGURE 9 it is observed that the resonator 23 is shownwith a voice coil circuit modified from that of FIGURE 6 by theinclusion therein of a series capacitor C. The effect of the capacitor Cis graphically illustrated in FIGURE 10 which is a series of phase shiftcharacteristics of the type shown in FIGURE 8, and shows theeifect ofvarious values of capacitance C in the circuit of FIGURE 9 with Rs heldat ten ohms. It is observed that increase in capacitance causes thein-phase frequency to shift downward, a capacitance of 75 microfaradseffecting a twenty cycle downward shift so that at 63 cycles there stillexists a significant in phase component which augments the bass responsewhereas without the capacitor C there is no such in phase componentsince th ephase shift has reached 90. Below 63 cycles there is an actualoutphasing condition without the capacitor which causes the response .todrop off quite rapidly. As in the case of phase line 40 of FIGURE 8reduction of Rs lowers the slope of the phase lines of FIGURE 10, andlistening tests confirm a marked increase of fundamentals in the bassfrequency output be tween 40 cycles and cycles. No audible differencewas discernible in the output above 100 cycles with the capacitor C inthe circuit or short circuited.

The effect of the capacitor as a phase shifting element is in convertingthe voice coil circuit from an inductive circuit exhibiting a currentlag due to the inductance L of the voice coil to a capacitive circuit inwhich the induced current can build up rapidly. In the previouslymentioned case comparing a short circuited capacitance with one of 75microfarads it is seen from FIGURE 10 that a desired phase shift ofalmost 40 is achieved. Effective resonator action is achieved in thedescribedsystem at frequencies substantially below the resonance rangeof the system because of the tight coupling between the cones of thedriver 22 and resonator 23 due to the acoustical stiffness of theenclosure at low frequencies resulting from the very small enclosed airvolume. This is not true however for frequencies substantially above thesystem resonance. It should be noted that independently of phaseconsiderations the electrodynamic damping is always operative becausemotion of the resonator cone produces a current flow and powerdissipation as earlier set forth.

Phase shifts in the direction opposite to that produced by seriescapacitance C may be caused by using a series inductor L in place of thecapacitor C, as shown in dashed line in FIGURE 9 and resulting in phaselines as generally indicated in FIGURE 10 by dashed line L"=a. Forspecial purposes combination inductionalcapacitance networks might beutilized.

, FIGURE 11 shows lines of constant phase shift plotted as a function offrequency and series capacitance from which other phase lines may beconstructed in FIGURE 10 for particular values of capacitance not shownthereon.

If desired, a plurality of dynamically damped resonators may beutilized, and these resonators may be of the same size or differentsizes, and of the same or different size than the driver. Moreover, eachmay be selected or designed to have a free air cone resonance frequencyeither the same as or different from that of the driver and otherresonators. Additionally, they may be selectively electrically tuned byutilizing different amounts of external series resistance in their voicecoil circuits, with or without reactive elements of selected reactancemagnitudes.

Having now described our invention in connection with particularlyillustrated embodiments thereof, variations and modifications thereofmay now occur from time to time to those persons normally skilled in theart without departing from the essential scope or spirit of ourinvention, and accordingly it is intended to claim the same broadly aswell as specifically as indicated by the appended claims.

What is claimed as new and useful is:

1. In a system for radiating sound energy, first vibratory means forpropagating sound energy and means for coupling the same to an actuatingsource, a baffle for said vibratory means, frequency selective secondvibratory means acoustically coupled to said first vibratory means, andelectrodynamic means electromechanically coupled to said secondvibratory means for damping the amplitude excursions of said firstvibratory means by selectively absorbing the sound energy therefrom,said electrodynamic damping means including means for selectivelyvarying within limits the relative phase relation between the motions ofsaid first and second vibratory means.

2. In a system for radiating sound energy, first vibratory'means forpropagating sound energy and means for coupling the same to an actuatingsource, a baffie for said vibratory means, second vibratory meansacoustically coupled to said first vibratory means, electrodynamic meanselectromechanically coupled to said second vibratory means for dampingthe amplitude excursions of said first vibratory means by selectivelyabsorbing the sound energy therefrom, and means electromechanicallycoupled to said second vibratory means for selectively varying Withinlimits the relative phase relation between the motions of said first andsecond vibratory means.

3. In a system for radiating sound energy, first vibratory means forpropagating sound energy and means for coupling the same to an actuatingsource, a batile for said vibratory means, second vibratory meansacoustically coupled to said first vibratory means, velocity responsivemeans electromechanically coupled to said second vibratory means fordamping the amplitude excursions of said first vibratory means byselectively absorbing the sound energy therefrom, and meanselectromechanically coupled to said second vibratory means forselectively varying within limits the relative phase relation betweenthe motions of said first and second vibratory means.

4. In a system for radiating sound energy, first vibratory means forpropagating sound energy and means for coupling the same to an actuatingsource, second vibratory means, an enclosure having apertured wallpartions to which both of said vibratory means are mounted so that onesurface of each of said vibratory means is contacted by the atmosphereoutside of said enclosure and the vibratory means are acousticallycoupled to one another Within the enclosure by means of the air volumecontained within the enclosure, each of said vibratory means beingcharacterized by a free air vibratory resonance frequency substantiallylower than the resonance frequency range of said vibratory means mountedin the enclosure, and velocity responsive damping meanselectromechanically coupled to said second vibratory means for dampingthe amplitude excursions of said first vibratory means by selectivelyabsorbing the sound energy therefrom.

5. In a system for radiating sound energy, first vibratory means forpropagating sound energy and means for coupling the same to an actuatingsource, second vibratory means, an enclosure having apertured wallportions to which both of said vibratory means are mounted so that onesurface of each of said vibratory means is contacted by the atmosphereoutside of said enclosure and the vibratory means are acousticallycoupled to one another within the enclosure by means of the air volumecontained within the enclosure, each of said vibratory means beingcharacterized by a free air vibratory resonance frequency substantiallylower than the resonance frequency range of said vibratory means mountedin the enclosure, and velocity responsive damping meanselectromechanically coupled to said second vibratory means for dampingthe amplitude excursions of said first vibratory means by selectivelyabsorbing the sound energy therefrom, said velocity responsive dampingmeans including means for selectively controlling within limits thedegree of damping desired.

6. In a system for radiating sound energy, first vibratory means forpropagating sound energy and means for coupling the same to an actuatingsource, second vibratory means, an enclosure having apertured wallportions to which both of said vibratory means are mounted so that onesurface of each of said vibratory means is contacted by the atmosphereoutside of said enclosure and the vibratory means are acousticallycoupled to one another within the enclosure by means of the air volumecontained within the enclosure, each of said vibratory means beincharacterized by a free air vibratory resonance frequency substantiallylower than the resonance frequency range of said vibratory means mountedin the enclosure, and velocity responsive damping meanselectromechanically coupled to said second vibratory means for dampingthe amplitude excursions of said first vibratory means by selectivelyabsorbing the sound energy therefrom, said velocity responsive dampingmeans including means for selectively varying Within limits the relativephase relation between the motions of said first and second vibratorymeans.

7. In a system for radiating sound energy, first and second loudspeakerseach of which has a vibratile diaphragm, an enclosure housing saidloudspeakers with the latter mounted to said enclosure so that one sideof the diaphragm of each of said loudspkears faces out of the enclosurethrough aperture means in the enclosure and the other sides of thediaphragms are acoustically coupled to one another by the air volumewithin the enclosure, means for coupling said first loudspeaker to anactuating source, said second loudspeaker being of the magnetic fieldtype and having a voice coil coupled to its vibratile diaphragm so thatthe turns of the voice coil are movable transversely to the lines ofmagnetic induction of the field, and means closing the voice coilcircuit to permit an induced current to flow therethrough whenever thevibratile diaphragm of said second loudspeaker is set in motion.

8. The system for radiating sound energy as set forth in claim 7 whereinsaid means closing the voice coil circuit to permit an induced currentto fiow therethrough includes an electrical resistor.

9. The system for radiating sound energy as set forth in claim 7 whereinsaid means closing the voice coil circuit to permit an induced currentto flow therethrough includes a selectively variable electricalresistor.

10. The system for radiating sound energy as set forth in claim 7wherein said means closing the voice coil circuit to permit an inducedcurrent to flow therethrough includes a capacitor.

11. The system for radiating sound energy as set forth in claim 7wherein said means closing the voice coil circuit to permit an inducedcurrent to flow therethrough includes an inductor.

12. In a system for radiating sound energy, first and secondloudspeakers each of which has a vibratile diaphragm, an enclosurehousing said loudspeakers with the latter mounted to said enclosure sothat one side of the diaphragm of each of said loudspeakers faces out ofthe enclosure through aperture means in the enclosure and the othersides of the diaphra-gms are acoustically coupled to one another by theair volume within the enclosure, the air volume Within the enclosurebeing sufficiently small to raise the resonant frequency range of thesystem substantially above the free air resonant frequency of saidsecond loudspeaker to thereby increase the acoustical stiffness of theenclosure and the acoustical coupling between the said vibratilediaphragms below the resonant frequency range of the system, means forcoupling said first loudspeaker to an actuating source, said secondloudspeaker being of the magnetic field type and having a voice coilcoupled to its vibratile diaphragm so that the turns of the voice coilare movable transversely to the lines of magnetic induction of thefield, and means closing the voice coil circuit to permit an inducedcurrent to flow therethrough whenever the vibratile diaphragm of saidsecond loudspeaker is set in motion.

'13. The system for radiating sound energy as set forth in claim 12wherein said means closing the voice coil circuit to permit an inducedcurrent to flow therethrough includes an electrical resistor.

14. The system for radiating sound energy as set forth in claim :12wherein said means closing the voice coil circuit to permit an inducedcurrent to flow therethrough includes a capacitor.

References Cited by the Examiner UNITED STATES PATENTS 1,734,944 ll/1929Green 179180 1,988,250 1/1935 Olson 179l16 X 2,489,862 11/1949 Cook179--180 3,202,773 8/1965 Tichy l79-18O KATHLEEN H. OLAEFY, PrimaryExaminer. WALTER L. LYNDE, Examiner. S. H. BOYER, L. A. WRIGHT,Assistant Examiners.

6. IN A SYSTEM FOR RADIATING SOUND ENERGY, FIRST VIBRATORY MEANS FORPROPAGATING SOUND ENERGY AND MEANS FOR COUPLING THE SAME TO AN ACTUATINGSOURCE, SECOND VIBRATORY MEANS, AN ENCLOSURE HAVING APERTURED WALLPORTIONS TO WHICH BOTH OF SAID VIBRATORY MEANS ARE MOUNTED SO THAT ONESURFACE OF EACH OF SAID VIBRATORY MEANS IS CONTACTED BY THE ATMOSPHEREOUTSIDE OF SAID ENCLOSURE AND THE VIBRATORY MEANS ARE ACOUSTICALLYCOUPLED TO ONE ANOTHER WITHIN THE ENCLOSURE BY MEANS OF THE AIR VOLUMECONTAINED WITHIN THE ENCLOSURE, EACH OF SAID VIBRATORY MEANS BEINGCHARACTERIZED BY A GREE AIR VIBRATORY RESONANCE FREQUENCY SUBSTANTIALLYLOWER THAN A RESONANCE FREQUENCY RANGE OF SAID VIBRATORY MEANS MOUNTEDIN THE ENCLOSURE, AND VELOCITY, RESPONSIVE DAMPING MEANSELECTROMECHANICALLY COUPLED TO SAID SECOND VIBRATORY MEANS FOR DAMPINGTHE AMPLITUDE EXCURSIONS OF SAID FIRST VIBRATORY MEANS BY SELECTIVELYABSORBING THE SOUND ENERGY THEREFROM, SAID VELOCITY RESPONSIVE DAMPINGMEANS INCLUDING MEANS FOR SELECTIVELY VARYING WITHIN LIMITS THE RELATIVEPHASE RELATION BETWEEN THE MOTIONS OF SAID FIRST AND SECOND VIBRATORYMEANS.